1. Field of the Invention
The present invention relates to a field effect transistor for detecting ionic material and a method of detecting ionic material using the field effect transistor.
2. Description of the Related Art
Transistor base biosensors including transistors are sensors that electrically detect ionic material, for example, biomolecules. Biosensors are manufactured using semiconductor processes and relatively quickly convert electric signals. Therefore, research on biosensors has widely progressed.
U.S. Pat. No. 4,238,757 discloses the measuring of biological reactions using a field effect transistor (“FET”) and relates to a biosensor capable of identifying an antigen-antibody reaction by detecting a current that varies due to a change in the surface charge concentration of a semiconductor inversion layer. This patent is directed toward a biosensor for detecting proteins. In U.S. Pat. No. 4,777,019, biological monomers are adsorbed onto the surface of a gate, and hybridization between the biological monomers and complementary monomers is measured using a FET.
U.S. Pat. No. 5,846,708 discloses a method of identifying hybridization using a charged coupled device (“CCD”). In this method, hybridization can be identified using a phenomenon of bonded biomolecules absorbing light. In U.S. Pat. Nos. 5,466,348 and 6,203,981, a circuit comprising a thin film transistor (“TFT”) is used and a signal-to-noise ratio is improved.
A FET used as a biosensor lowers costs and requires less time than other conventional methods. In addition, a FET can be easily applied to integrated circuits (“IC”)/microelectrical mechanical systems (“MEMS”) processes.
FIG. 1 is a schematic diagram of a conventional FET for detecting ionic material. Referring to FIG. 1, a source 12 and a drain 13 are respectively formed in side portions of a substrate 11 doped with an n-or p-type material. The source 12 and the drain 13 are doped with an opposite conductivity type to that of the substrate 11. A channel 15 is interposed between the source 12 and the drain 13 and an insulating layer 14 contacting the source 12 and the drain 13 is formed on the substrate 11. A reference electrode 16 is formed above the insulating layer 14 and a constant voltage is applied to the reference electrode 16.
The size of the sensor including the FET is on the scale of microns, while the size of the reference electrode 16 is on the scale of millimeters. Therefore, it is difficult to reduce the overall size of the sensor.
On the other hand, U.S. Pat. No. 4,269,682 discloses a system including a measuring unit and a reference ejectrode, both having an insulating gate FET, and a pseudo reference electrode which applies bias to each FET and immobilizes electric potential of an electrolyte solution.
In the case of a metal electrode to which the system above is applied, voltage is very unstable. Therefore, in order to eliminate noise, gradation measurement may be performed using a reference FET.
Conventional FETs for detecting ionic material have relatively large scattering during their manufacture. Therefore, when similar samples are detected using these FETs, there is wide variation in measured electrical values. Accordingly, electrical values are measured using a large number of array FETs and then, the values are filtered and averaged to select and use good data.